The present invention relates to a color detecting circuit which is favorably applied to color correction system of an image processing circuit of a television receiver or the like.
In the image processing, correction of color is frequently required. For example, in a television receiver or the like, because of relation of phase and gain of the demodulation axis being set and difference of the light emission spectrum of each fluorescent material of a cathode ray tube, emission of some specific color may become difficult.
Method of correcting only the hue of the specific region is as follows. That is, a specific color signal, for example, negative component (-(R-Y)) of (R-Y) signal is extracted. On the other hand, signal level of a color signal other than (R-Y), for example, negative component (-(B-Y)) of (B-Y) signal is extracted. In response to the signal level, the gain control of the negative component (-(R-Y)) of the specific color signal extracted as above described is performed. The negative component (-(R-Y)) of the prescribed color signal subjected to the gain control is added to the prescribed color signal (R-Y), thereby only the hue of the specific region can be corrected. In this example, when skin color is adjusted accurately, green color inclined to yellow green is corrected to color closer to green. Since the green color is inclined to yellow green, only component of (R-Y) in the negative (-) direction is amplified. In this case, in order to reduce the influence to other color, since the amplification factor (gain) of the negative component of (R-Y) is controlled in response to the signal level of the negative component of (B-Y), only the hue in some range among the hue of the third quadrant can be varied.
In this method, however, problems exist as follows.
In a first problem, since the color correction is performed in proportion to the amplitude of (R-Y) and the amplitude of (B-Y), the color correction is performed even at color with small amplitude, i.e., with low color saturation degree. In the case of color with low color saturation degree, since the noise component such as remaining carrier or color noise is predominant over the color component, the noise is emphasized by the color correction.
In a second problem, an output signal of an amplifier to control the gain becomes a specific color detection signal, for example, the negative component (-(R-Y)), and this color signal is added to the specific color signal (R-Y) or not added thereto, thereby the color correction can be turned on or off. However, if the output offset exists in the amplifier to control the gain, the DC potential of the (R-Y) output fluctuates by the ON/OFF operation of the color correction and therefore the white balance of the image fluctuates. This problem can be dealt with by adjustment of absorbing the output offset, but the adjustment is necessary.
In a third problem, the hue correction range becomes the phase range from the orthogonal phase of the (R-Y) demodulation phase to the orthogonal phase of the (B-Y) demodulation phase. That is, the phase range to enable the hue correction is determined with uniqueness by the demodulation phase of the color signal and arbitrary range cannot be set.
In a fourth problem, the negative component of (R-Y) is increased and the (R-Y) component is decreased in the hue correction range, thereby the (G-Y) component is relatively increased and the emission of green is corrected. That is, since the color correction is of hue correction type, the color correction of the specific color with the low color saturation degree is not taken in consideration and therefore the color saturation degree cannot be corrected.